Individuals with impaired hearing often experience difficulty understanding conversational speech in background noise. What has not heretofore been well understood is that the majority of daily conversations occur in background noise of one form or another. In some cases, the background noise may be more intense than the target speech, resulting in a severe signal-to-noise ratio problem. In a study of this signal-to-noise problem, Preasons et al, "Speech levels in various environments," Bolt Beranek and Newman report No. 3281, Washington, D.C., October 1976, placed a head-worn microphone and tape recorder on several individuals and sent them about their daily lives, obtaining data in homes, automobiles, trains, hospitals, department stores, and airplanes. They found that nearly 1/4 of the recorded conversations took place in background noise levels of 60 dB sound pressure level (SPL) or greater, and that nearly all of the latter took place with a signal-to-noise ratio between -5 dB and +5 dB. (A signal-to-noise ratio of -5 dB means the target speech is 5 dB less intense than the background noise.) As discussed in a review by Mead Killion, "The Noise Problem: There's hope," Hearing Instruments Vol. 36, No. 11, 26-32 (1985), people with normal hearing can carry on a conversation with a -5 dB signal-to-noise ratio, but those with hearing impairment generally require something like +10 dB. Hearing impaired individuals are thus excluded from many everyday conversations unless the talker raises his or her voice to an unnatural level. Moreover, the evidence of Carhart and Tillman, "Interaction of competing speech signals with hearing losses," Archives of Otolaryngology, Vol. 91, 273-9 (1970), indicates that hearing aids made the problem even worse. More recent studies by Hawkins and Yacullo, "Signal-to-noise ratio advantage of binaural hearing aids and directional microphones under different levels of reverberation," J. Speech and Hearing Disorders, Vol. 49, 278-86 (1984), have shown that hearing aids can now help, but still leave the typical hearing aid wearer with a deficit of 10-15 dB relative to a normal-hearing person's ability to hear in noise.
One approach to the problem is the use of digital signal processors such as described in separate papers by Harry Levitt and Birger Kollmeier at the 15th Danavox Symposium "Recent development in hearing instrument technology," Scanticon, Kolding, Denmark, March 30 through Apr. 2, 1993 (to be published as the Proceedings of the 15th Danavox Symposium). This approach, using multiple microphones and high-speed digital processors, provide a few dB improvement in signal-to-noise ratio. The approach, however, requires very large research expenditures, and, at present, large energy expenditures. It is estimated that the processor described by Levitt would require 40,000 hearing aid batteries per week to keep it powered up. One of the approaches described by Kollmeier operated at 400 times slower than real time, indicating 400 SPARC processors operating simultaneously would be required to obtain real-time operation, for an estimated expenditure of 60,000 hearing aid batteries per hour. Such digital signal processing schemes therefore hold little immediate hope for the hearing aid user.
First-order directional microphones have been used in behind-the-ear hearing aids to improve the signal-to-noise ratio by rejecting a portion of the noise coming from the sides and behind the listener. Carlson and Killion, "Subminiature directional microphones", J. Audio Engineering Society, Vol. 22, 92-6 (1974), describe the construction and application of such a subminiature microphone suitable for use in behind-the-ear hearing aids. Hawkins and Yacullo (see above) found that such a microphone could improve the effective signal-to-noise ratio by 3-4 dB.
First-order directional microphones, however, are not without their drawbacks when utilized in the in-the-ear hearing aids employed by some 75% of hearing aid wearers. The experimental sensitivity of a first-order directional microphone is typically 6-8 dB less when mounted in an in-the-ear hearing aid compared to its sensitivity in a behind-the-ear mounting. These results come about because of the shortened distance available inside the ear and the effect of sound diffraction about the head and ear. An additional problem with directional microphones in head-worn applications is that the improvement they provide over the normal omni-directional microphone is less than occurs in free-field applications because the head and pinna of the ear provide substantial directionality at high frequencies. Thus in both behind-the-ear and in-the-ear applications, the directivity index (ratio of sensitivity to sound from the front to the average sensitivity to sounds from all directions) might be 4.8 dB for a first-order directional microphone tested in isolation and 0 dB for an omnidirectional microphone tested in isolation. When mounted on the head, however, the omnidirectional microphone might have a directivity index of 3 dB at high frequencies and the directional microphone perhaps 5.5 dB. As a result, the improvement in the head-mounted case is 2.5 dB.
An approach exploiting microphone directional sensitivity was pursued by Wim Soede. That approach utilizes 5-microphone directional arrays suitable for head-worn applications. The array and its theoretical description are described in his Ph.D. dissertation "Development and evaluation of a new directional hearing instrument based on array technology," Gebotekst Zoetermeet/1990, Delft University of Technology, Delft, The Netherlands. The array provided a directivity index of 10 dB or greater. The problem with this array approach is that the Soede array is 10 cm long, requiring eyeglass-size hearing aids. It is certainly not practical for the in-the-ear hearing aids most often used in the United States. While there may be many individuals whose loss is so severe that the improved signal-to-noise obtained with such a head-worn array would make it attractive, a majority of hearing aid wearers would find the size of the array unattractive.
Second-order directional microphones are more directionally sensitive than their first order counterparts. Second-order directional microphones, however, have always been considered impractical because their sensitivity is so low. The frequency response of a first-order directional microphone falls off at 6 dB/octave below about 2 kHz. The frequency response of a second-order directional microphone falls off at 12 dB/octave below about 2 kHz. At 200 Hz, therefore, the response of a second-order directional microphone is 40 dB below that of it's comparable omni-directional microphone. If electrical equalization is used to restore the low-frequency response, the amplified microphone noise will be 40 dB higher. The steady hiss of such amplified microphone noise is objectionable in a quiet room, and hearing aids with equivalent noise levels more than about 10-15 dB greater than that obtained with an omni-directional microphone have been found unacceptable in the marketplace. For similar reasons, first order microphones have likewise not gained wide acceptance for use in hearing aids.